Do you know if Hugh had any siblings?
On the 1851 census for Neilston there is a Donald McDonald aged 31, sawer born Inverness. His wife Mary, 24, born Barrhead, and a daughter Margaret aged 1 born Barrhead.
I still can't find a Hugh on the census or birth records. If Hugh and Catherine followed the usual scottish naming convention the first boy would be named after the paternal grandfather and the first girl after the maternal grandmother - hence why I thought I would look for a Donald McDonald in Barrhead.
Donald McDonald and Mary (McCallum) seem to have had a large family and in 1861 I think they are in Greenock, but I still can't see Hugh.
Sorry this doesn't help much!